DISSOLVED ORGANIC NITROGEN CYCLING IN HYPORHEIC ZONES OF HEADWATER STREAMS

Headwater streams retain and transform nutrients entering from terrestrial ecosystems and thus may be critical to regulating nutrient export to rivers and estuaries. Groundwater-surface water exchange may be a key driver of biogeochemical processes in headwater streams where flow paths of water and nutrients are predominantly from hillslope to stream. Biogeochemical theory has not addressed stream nutrient cycling, yet recent studies suggest that hyporheic zones may be important to understanding catchment-scale controls on nutrient export. In particular, hyporheic zones can strongly influence stream retention of dissolved inorganic nitrogen (DIN). However, dissolved organic nitrogen (DON) dominates total N flux from many headwater catchments, yet little is known about the role of hyporheic zones in DON cycling. As part study addressing DON cycling in streams, surface water and hyporheic N pools were quantified in headwater streams spanning wide atmospheric deposition (5 to 30 kg N/ha/yr) and forest successional (90 to >400 yr old) gradients in the Appalachian Mts., USA. DON concentrations were higher in hyporheic waters (90 to 350 ppb) than surface waters (32 to 215 ppb) at all sites. Hyporheic N pools were dominated by DON at low N-deposition sites and by DIN at high N-deposition sites. In addition, hyporheic ammonium-N pools were higher in streams draining old growth than aggrading forests. Current experiments address the biological utilization of DON and DIN in headwater hyporheic zones to assess the contribution of groundwater-surface water exchange to catchment-scale N transformation and retention.